Final Fantasy VII Advent Children is a glorified tech demo. Its plot is completely incomprehensible without having played Final Fantasy VII first, and even then it is frustratingly vague. All of Cloud's character development from the game had regressed, new villains are introduced out of nowhere with no solid explanation of their backgrounds, seemingly dead characters are revived for no apparent reason, and Sephiroth is resurrected for absolutely no good reason.
The dialogue is horrendously bad, the characterization is non-existent and what is
comprehensible in the plot is riddled with massive plot holes.
The up side? The movie looks and sounds fantastic with ridiculously good CGI visuals and a great soundtrack by Nobuo Uematsu. Really, the movie is nothing but a giant, lovingly assembled pile of Fan Service
. If you are a fan of Final Fantasy VII, and that's what you want, you will be happy. If you want a storyline that can stand on its own two feet, makes sense, has strong characterization and adds anything meaningful to the already existing storyline of Final Fantasy VII, you will be incredibly disappointed. In other words, the movie exists solely for Rule Of Cool
. If you are Okay with that, you will enjoy it. If you are not, don't bother.
Oh, yeah. Any scene that doesn't involve over the top action is filled with either nonsense exposition or wall-to-wall Wangst
If none of that scares you off, have fun.